Control of selective catalytic reduction in heavy-duty motor vehicle engines

ABSTRACT

A heavy duty truck includes a diesel engine that generates an exhaust gas flow and an exhaust after-treatment system for treatment of the exhaust gas flow. The exhaust after-treatment system includes at least one temperature sensor at an underbody SCR system within the exhaust after-treatment system and a DEF injector upstream of a close-coupled SCR system within the exhaust after-treatment system. The DEF injector is operated to inject DEF into the exhaust gas flow at a rate that varies as a function of a temperature measured by the temperature sensor.

BACKGROUND Technical Field

The present disclosure relates generally to control of selectivecatalytic reduction in heavy-duty motor vehicle engines and tomodulation of performance of selective catalytic reduction systems inheavy-duty motor vehicle exhaust after-treatment systems.

Description of the Related Art

Regulated emissions from today's heavy-duty engines demand very lowlevels of tailpipe emissions, and standards are expected to be furtherreduced in the near future. To reduce tailpipe exhaust emissions,current technologies rely on aggressive engine control strategies andexhaust after-treatment catalyst systems (catalyst systems used to treatengine exhaust are referred to herein as exhaust after-treatmentsystems, emissions after-treatment systems, or EAS). The EAS for atypical heavy-duty diesel or other lean-burning engine may include adiesel oxidation catalyst (DOC) to oxidize unburned fuel and carbonmonoxide, a diesel particulate filter (DPF) for control of particulatematter (PM), selective catalytic reduction (SCR) systems for reductionof oxides of nitrogen (NO_(x)), and/or an ammonia oxidation catalyst(AMOX). Performance of EAS systems, and of SCR systems in particular, isdependent upon exhaust gas temperature and other parameters.

SCR processes use catalysts to catalyze the NO_(x) reduction and a fluidreferred to as DEF (diesel emission fluid), which acts as a NO_(x)reductant over the SCR catalyst. DEF is an aqueous solution thatevaporates and decomposes to chemically release ammonia so that theammonia is available for reaction. Efficiency of SCR operation isdependent upon temperature. For example, DEF evaporation anddecomposition is dependent upon temperature, with higher temperatures(e.g., temperatures over 150, 160, 170, 180, 190, 200, 250, 300, or 350degrees Celsius) generally improving performance. Temperature levelsrequired to ensure compliance with emissions regulations may be highlydependent upon a wide variety of variables and are in some casesdetermined experimentally for specific engines, trucks, and operatingconditions thereof. Thus, an EAS may include a heater to increase thetemperature of the exhaust, to facilitate DEF injection, evaporation,and decomposition at rates sufficient to allow efficient performance ofthe SCR processes.

BRIEF SUMMARY

A method may be summarized as comprising: operating a diesel engine of aheavy-duty truck such that the diesel engine generates an exhaust gasflow that enters an exhaust after-treatment system of the heavy-dutytruck, the exhaust after-treatment system including a close-coupledselective catalytic reduction system and an underbody selectivecatalytic reduction system downstream of the close-coupled selectivecatalytic reduction system with respect to the exhaust gas flow;monitoring a temperature of the exhaust gas flow at the underbodyselective catalytic reduction system; and controlling a DEF injectorupstream of the close-coupled selective catalytic reduction system toinject DEF into the exhaust gas flow at a rate that varies as a functionof the monitored temperature across a range of at least 25 degreesCelsius in the monitored temperature.

Controlling the DEF injector may include controlling the DEF injector toinject DEF into the exhaust gas flow at a rate that varies as a functionof the monitored temperature across a range of at least 50 degreesCelsius in the monitored temperature. Controlling the DEF injector mayinclude controlling the DEF injector to inject DEF into the exhaust gasflow at a rate that varies as a function of the monitored temperatureacross a range of at least 100 degrees Celsius in the monitoredtemperature.

The method may further comprise controlling a DEF injector downstream ofthe close-coupled selective catalytic reduction system and upstream ofthe underbody selective catalytic reduction system to inject DEF intothe exhaust gas flow at a rate that varies as a function of themonitored temperature. Controlling the DEF injector downstream of theclose-coupled selective catalytic reduction system and upstream of theunderbody selective catalytic reduction system may include operating theDEF injector downstream of the close-coupled selective catalyticreduction system and upstream of the underbody selective catalyticreduction system to reduce NO_(x) levels in the exhaust gas flow toensure compliance with emissions regulations. Controlling the DEFinjector upstream of the close-coupled selective catalytic reductionsystem and controlling the DEF injector downstream of the close-coupledselective catalytic reduction system and upstream of the underbodyselective catalytic reduction system may include optimizing a divisionlabor of reducing NO_(x) levels to comply with emissions regulations.

The DEF injector may inject DEF into the exhaust gas flow at a rate thatdecreases as the monitored temperature increases. The DEF injector mayinitially inject DEF into the exhaust gas flow at a rate sufficient forthe close-coupled selective catalytic reduction system to reduce NO_(x)levels to comply with emissions regulations. After the DEF injectorinjects DEF into the exhaust gas flow at the rate sufficient for theclose-coupled selective catalytic reduction system to reduce NO_(x)levels to comply with emissions regulations, the DEF injector may injectDEF into the exhaust gas flow at a lower rate sufficient for theclose-coupled selective catalytic reduction system to reduce NO_(x)levels halfway to compliance with emissions regulations.

The method may further comprise, while the DEF injector upstream of theclose-coupled selective catalytic reduction system injects DEF into theexhaust gas flow at the rate sufficient for the close-coupled selectivecatalytic reduction system to reduce NO_(x) levels halfway to compliancewith emissions regulations, controlling a DEF injector downstream of theclose-coupled selective catalytic reduction system and upstream of theunderbody selective catalytic reduction system to inject DEF into theexhaust gas flow at a rate sufficient for the underbody selectivecatalytic reduction system to reduce NO_(x) levels to comply withemissions regulations. After the DEF injector injects DEF into theexhaust gas flow at the lower rate sufficient for the close-coupledselective catalytic reduction system to reduce NO_(x) levels halfway tocompliance with emissions regulations, the DEF injector may cease toinject DEF into the exhaust gas flow. The method may further comprise,once the DEF injector upstream of the close-coupled selective catalyticreduction system ceases to inject DEF into the exhaust gas flow,controlling a DEF injector downstream of the close-coupled selectivecatalytic reduction system and upstream of the underbody selectivecatalytic reduction system to inject DEF into the exhaust gas flow at arate sufficient for the underbody selective catalytic reduction systemto reduce NO_(x) levels to comply with emissions regulations.

The method may further comprise monitoring a NO_(x) level upstream ofthe close-coupled selective catalytic reduction system. Controlling theDEF injector may include operating the DEF injector to inject DEF intothe exhaust gas flow at a rate that varies as a function of themonitored NO_(x) level. Controlling the DEF injector may includeoperating the DEF injector to inject DEF into the exhaust gas flow toachieve a target ammonia-to-NO_(x) ratio in the close-coupled selectivecatalytic reduction system.

A method may be summarized as comprising: operating a diesel engine of aheavy-duty truck such that the diesel engine generates an exhaust gasflow that enters an exhaust after-treatment system of the heavy-dutytruck, the exhaust after-treatment system including a close-coupledselective catalytic reduction system and an underbody selectivecatalytic reduction system downstream of the close-coupled selectivecatalytic reduction system with respect to the exhaust gas flow;monitoring a temperature of the exhaust gas flow at the underbodyselective catalytic reduction system; and controlling a DEF injectorupstream of the close-coupled selective catalytic reduction system toinject DEF into the exhaust gas flow at a rate that varies as a functionof the monitored temperature over a span of time of at least thirtyseconds, at least one minute, or at least two minutes.

A heavy-duty truck may be summarized as comprising: a diesel engine; anexhaust after-treatment system having an upstream end and a downstreamend opposite the upstream end, the upstream end coupled to the dieselengine, the exhaust after-treatment system including a close-coupledselective catalytic reduction system and an underbody selectivecatalytic reduction system downstream of the close-coupled selectivecatalytic reduction system; and an engine control unit configured to:operate the diesel engine such that the diesel engine generates anexhaust gas flow that enters the exhaust after-treatment system; monitora temperature of the exhaust gas flow at the underbody selectivecatalytic reduction system; and control a DEF injector upstream of theclose-coupled selective catalytic reduction system to inject DEF intothe exhaust gas flow at a rate that varies as a function of themonitored temperature across a range of at least 25 degrees Celsius inthe monitored temperature. The engine control unit may be configured tocontrol the DEF injector to inject DEF into the exhaust gas flow toachieve a target ammonia-to-NO_(x) ratio in the close-coupled selectivecatalytic reduction system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagram of an exhaust after-treatment systemincluding a DOC, a DPF, and dual SCR systems.

FIG. 2 illustrates results of experimental testing of a heavy-dutyvehicle including a diesel engine and the exhaust after-treatment systemof FIG. 1.

FIG. 3 illustrates additional results of experimental testing of aheavy-duty vehicle including a diesel engine and the exhaustafter-treatment system of FIG. 1.

FIG. 4 illustrates a flow chart of a method of using the systemsdescribed herein.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various disclosedembodiments. However, one skilled in the relevant art will recognizethat embodiments may be practiced without one or more of these specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures associated with the technology have notbeen shown or described in detail to avoid unnecessarily obscuringdescriptions of the embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments. Also, as used in thisspecification and the appended claims, the singular forms “a,” “an,” and“the” include plural referents unless the context clearly dictatesotherwise. It should also be noted that the term “or” is generallyemployed in its sense including “and/or” unless the context clearlydictates otherwise.

The use of ordinals such as first, second and third does not necessarilyimply a ranked sense of order, but rather may only distinguish betweenmultiple instances of an act or structure.

Terms of geometric alignment may be used herein. Any components of theembodiments that are illustrated, described, or claimed herein as beingaligned, arranged in the same direction, parallel, or having othersimilar geometric relationships with respect to one another have suchrelationships in the illustrated, described, or claimed embodiments. Inalternative embodiments, however, such components can have any of theother similar geometric properties described herein indicating alignmentwith respect to one another. Any components of the embodiments that areillustrated, described, or claimed herein as being not aligned, arrangedin different directions, not parallel, perpendicular, transverse, orhaving other similar geometric relationships with respect to oneanother, have such relationships in the illustrated, described, orclaimed embodiments. In alternative embodiments, however, suchcomponents can have any of the other similar geometric propertiesdescribed herein indicating non-alignment with respect to one another.

Various examples of suitable dimensions of components and othernumerical values may be provided herein. In the illustrated, described,and claimed embodiments, such dimensions are accurate to within standardmanufacturing tolerances unless stated otherwise. Such dimensions areexamples, however, and can be modified to produce variations of thecomponents and systems described herein. In various alternativeembodiments, such dimensions and any other specific numerical valuesprovided herein can be approximations wherein the actual numericalvalues can vary by up to 1, 2, 5, 10, 15 or more percent from thestated, approximate dimensions or other numerical values.

As described herein, experiments may be performed and measurements maybe taken while an engine or a vehicle including an engine are operatingat “steady state.” As used herein, the term “steady state” may mean thatthe engine or the vehicle including the engine are operating with alloperating parameters, including engine speed, power level, etc.,unchanged or substantially unchanged over a period of time of at leastone, at least two, at least three, at least four, at least five, atleast six, or at least ten seconds.

Traditionally, heavy-duty vehicles included many components of exhaustafter-treatment systems “underbody,” that is, underneath the engine,cab, or another portion of the vehicle, where space is relatively freelyavailable and these components can therefore generally be larger thanwould otherwise be practical. Some modern heavy-duty vehicles, however,have begun to include a “close-coupled,” “up-close,” or “light-off” SCRunit much closer to the engine and exhaust ports thereof (e.g., adjacentto a turbine outlet of a turbocharger) and upstream of the traditionalunderbody exhaust after-treatment system, which can provide certainadvantages in that the temperature of the engine exhaust may be higherwhen it is closer to the engine, although locating an SCR unit nearerthe engine limits the available space and thus its practical size. Thus,some modern heavy-duty vehicles have included both a “close-coupled” SCRunit upstream with respect to the flow of the exhaust, such as adjacentto a turbine outlet of a turbocharger, to take advantage of the higherexhaust temperatures, as well as an “underbody” SCR unit downstream withrespect to the flow of the exhaust, such as under the engine or cab ofthe vehicle, to take advantage of the greater available space.

FIG. 1 illustrates a diagram of an exhaust after-treatment system 100that has a first, upstream end 102 and a second, downstream end 104opposite to the first, upstream end 102. The exhaust after-treatmentsystem 100 is a component of a vehicle, such as a large, heavy-duty,diesel truck, and in use carries exhaust from the diesel engine of thetruck to a tailpipe of the truck. For example, the first, upstream end102 of the exhaust after-treatment system 100 may be coupled directly toan exhaust port or an outlet port of the diesel engine, such as aturbine outlet of a turbocharger thereof, and the second, downstream end104 may be coupled directly to an inlet port of a tailpipe or muffler ofthe truck. Thus, when the engine is running and generating exhaust, theexhaust travels along the length of the exhaust after-treatment system100 from the first, upstream end 102 thereof to the second, downstreamend 104 thereof.

As illustrated in FIG. 1, the exhaust after-treatment system 100includes, at its first, upstream end 102, or proximate or adjacentthereto, a first temperature sensor 106, which may be a thermocouple, tomeasure the temperature of the exhaust gas flow as it leaves the engineand enters the exhaust after-treatment system 100, before heat begins tobe lost through the exhaust after-treatment system 100 to theenvironment. The exhaust after-treatment system 100 also includes, atits first, upstream end 102, or proximate or adjacent thereto, or justdownstream of the first temperature sensor 106, a first NO_(x) sensor108, to measure the content of NO_(x) gases in the exhaust gas flow asit leaves the engine and enters the exhaust after-treatment system 100.The exhaust after-treatment system 100 also includes, at its first,upstream end 102, or proximate or adjacent thereto, or just downstreamof the first NO_(x) sensor 108, a first DEF injector 110, to inject DEFinto the exhaust gas flow as it leaves the engine and enters the exhaustafter-treatment system 100.

The exhaust after-treatment system 100 may also include, proximate oradjacent its first, upstream end 102, or just downstream of the firstDEF injector 110, a first heater 112, which may be anelectrically-powered resistive heater or heating element, a burner, orany other suitable heater, to inject heat energy into the exhaust gasflow and the injected DEF as they flow through the exhaustafter-treatment system 100. The exhaust after-treatment system 100 alsoincludes, just downstream of the first heater 112, a second temperaturesensor 114, which may be a thermocouple, to measure the temperature ofthe exhaust gas flow as it leaves the first heater 112 and just beforeor just as it enters a first, close-coupled SCR system 116, or at theinlet to the close-coupled SCR system 116. The exhaust after-treatmentsystem 100 also includes, just downstream of the first heater 112 andthe second temperature sensor 114, the first, close-coupled SCR system116, which is configured to reduce oxides of nitrogen (NO_(x)) in theexhaust gas flow.

The exhaust after-treatment system 100 also includes, just downstream ofthe first SCR system 116, a third temperature sensor 136, which may be athermocouple, to measure the temperature of the exhaust gas flow as itleaves the first SCR system 116. In some implementations, the secondtemperature sensor 114 and the third temperature sensor 136 may becollectively referred to as an SCR bed temperature sensor. For example,a temperature of a catalytic bed of the first, close-coupled SCR system116 may be measured, calculated, estimated, or otherwise determinedbased on the measurements provided by the second temperature sensor 114and the third temperature sensor 136, such as by averaging thetemperature measurements provided by the second temperature sensor 114and the third temperature sensor 136.

The exhaust after-treatment system 100 also includes, just downstream ofthe first SCR system 116 and/or the third temperature sensor 136, asecond NO_(x) sensor 118, to measure the content of NO_(x) gases in theexhaust gas flow as it leaves the first SCR system 116. In practice, thefirst NO_(x) sensor 108 and the second NO_(x) sensor 118 can be usedtogether to monitor, assess, or measure the performance of the first SCRsystem 116. Together, the first temperature sensor 106, the first NO_(x)sensor 108, the first DEF injector 110, the first heater 112, the secondtemperature sensor 114, the first, close-coupled SCR system 116, thethird temperature sensor 136, and the second NO_(x) sensor 118 can bereferred to as a close-coupled portion of the exhaust after-treatmentsystem 100, as they can be collectively located at or adjacent to theengine of the vehicle.

The exhaust after-treatment system 100 also includes, downstream of thefirst SCR system 116, the third temperature sensor 136, and the secondNO_(x) sensor 118, a DOC component 120, to oxidize unburned fuel andcarbon monoxide in the exhaust gas flow. The exhaust after-treatmentsystem 100 also includes, downstream of the DOC component 120, a DPF122, to reduce or otherwise control particulate matter in the exhaustgas flow. The exhaust after-treatment system 100 also includes,downstream of the DPF 122, a fourth temperature sensor 130, which may bea thermocouple, to measure the temperature of the exhaust gas flow as itleaves the DPF 122. The exhaust after-treatment system 100 alsoincludes, downstream of the DPF 122, or just downstream of the fourthtemperature sensor 130, a second DEF injector 124, to inject DEF intothe exhaust gas flow as it leaves the DPF 122.

In some embodiments, the exhaust after-treatment system 100 may alsoinclude, just downstream of the fourth temperature sensor 130 and thesecond DEF injector 124, a mixer 132 and a second heater, which may bean electrically-powered resistive heater or heating element, a burner,or any other suitable heater, to inject heat energy into the exhaust gasflow and the injected DEF as they flow through the exhaustafter-treatment system 100. The exhaust after-treatment system 100 alsoincludes, just downstream of the mixer 132 and the second heater, afifth temperature sensor 134, which may be a thermocouple, to measurethe temperature of the exhaust gas flow as it leaves the second heaterand just before or just as it enters a second, underbody SCR system 126,or at the inlet to the underbody SCR system 126. The exhaustafter-treatment system 100 also includes, just downstream of the mixer132, the second heater, and the fifth temperature sensor 134, thesecond, underbody SCR system 126, which is configured to reduce oxidesof nitrogen (NO_(x)) in the exhaust gas flow.

The exhaust after-treatment system 100 also includes, just downstream ofthe second SCR system 126, a sixth temperature sensor 138, which may bea thermocouple, to measure the temperature of the exhaust gas flow as itleaves the second SCR system 126. In some implementations, the fifthtemperature sensor 134 and the sixth temperature sensor 138 may becollectively referred to as an SCR bed temperature sensor. For example,a temperature of a catalytic bed of the second, underbody SCR system 126may be measured, calculated, estimated, or otherwise determined based onthe measurements provided by the fifth temperature sensor 134 and thesixth temperature sensor 138, such as by averaging the temperaturemeasurements provided by the fifth temperature sensor 134 and the sixthtemperature sensor 138.

In some alternative embodiments, the exhaust after-treatment system 100may not include the second heater and may include only a single heater,i.e., the first heater 112, to reduce overall costs. Similarly, in someembodiments, the exhaust after-treatment system 100 may not include allof the temperature sensors described herein, such as the thirdtemperature sensor 136, fourth temperature sensor 130, fifth temperaturesensor 134, and/or sixth temperature sensor 138, such as to furtherreduce overall costs. In such implementations, such temperature sensorsmay be replaced by virtual temperature sensors, which may measure,calculate, estimate, simulate, or otherwise determine a temperature atthe same location, such as based on equations, data, simulations, and/ormodels of the behavior of temperatures at such locations under theoperating conditions of the systems described herein.

The exhaust after-treatment system 100 also includes, just downstream ofthe second SCR system 126 and/or the sixth temperature sensor 138, andat its second, downstream end 104, or proximate or adjacent thereto, athird NO_(x) sensor 128, to measure the content of NO_(x) gases in theexhaust gas flow as it leaves the second SCR system 126. In practice,the second NO_(x) sensor 118 and the third NO_(x) sensor 128 can be usedtogether to monitor, assess, or measure the performance of the secondSCR system 126. Together, the DOC component 120, the DPF 122, the secondDEF injector 124, the fourth temperature sensor 130, the mixer 132, thesecond heater, the fifth temperature sensor 134, the second SCR system126, the sixth temperature sensor 138, and the third NO_(x) sensor 128can be referred to as an underbody portion of the exhaustafter-treatment system 100, as they can be collectively locatedunderneath the engine, cab, or another portion of the vehicle.

As noted previously, performance of exhaust after-treatment systems, andof SCR systems in particular, is dependent upon exhaust gas temperature.More specifically, DEF evaporation and decomposition is dependent upontemperature, with higher temperatures generally improving performance.Thus, operation of a heater to increase the temperature of the exhaustgas flow can be critical to maintaining compliance with emissionsregulations. Nevertheless, operation of a heater to increase thetemperature of the exhaust gas flow naturally incurs a fuel penalty andthus a reduction of overall system fuel efficiency. Thus, it is criticalto ensure accurate and precise performance of such heaters, to ensurecompliance with emissions standards without unduly reducing overall fuelefficiency.

There are additional trade-offs involved in relying on the close-coupledSCR system 116 and/or the underbody SCR system 126 to reduce NO_(x)levels in the exhaust gas flow. For example, as noted previously, use ofthe close coupled SCR system 116 may be advantageous because the exhaustgas flow is ordinarily already naturally at a higher temperature than itis at the underbody SCR system 126 (ignoring operation of the firstheater 112 and/or the second heater), particularly under cold-startconditions and/or low load operation. Nevertheless, as also notedpreviously, the underbody SCR system 126 may be larger than theclose-coupled SCR system 116.

Furthermore, relative weighting of the burden of NO_(x) reductionbetween the close-coupled SCR system 116 and the underbody SCR system126 results in different levels of various tailpipe emissions. NO_(x)(oxides of nitrogen) in the exhaust gas flow include both NO (nitricoxide) and NO₂ (nitrogen dioxide), but diesel engine exhaust typicallyincludes NO_(x) predominantly in the form of NO rather than NO₂. As theexhaust gas flow passes across the DOC 120, however, NO is oxidized toNO₂, and as such, the underbody SCR system 126 has higher levels of NO₂than the close-coupled SCR system 116. As such, the close-coupled SCRsystem 116 is more heavily governed by the standard SCR reaction andother NO-based reactions than the underbody SCR system 126. Under coldconditions (e.g., SCR bed temperatures under 250 degrees Celsius),dosing of DEF in the presence of NO₂ can form nitrates, whichsubsequently form N₂O, which is a greenhouse gas. Under conditions whenthe underbody SCR system 126 is cold, therefore, it can be preferable toleverage the close-coupled SCR system 116. The control strategytherefore heavily weights operation of the close-coupled SCR system 116relative to operation of the underbody SCR system 126 under cold-startand low-load operation conditions.

Additionally, the DPF 122 includes a catalyst that traps soot (e.g.,black carbon) from the exhaust gas flow. The DPF 122 has a maximumcapacity that, once reached, requires active regeneration of the DPF 122to oxidize the soot to CO₂. Active regeneration is achieved by raisingthe temperature of the exhaust to greater than 500 degrees Celsius, andtherefore increases both fuel consumption and CO₂ emissions. Under warmconditions (e.g., exhaust temperature greater than 300 degrees Celsius),the soot in the DPF 122 can undergo passive regeneration using NO₂generated by the DOC 120. It is desirable to maximize passiveregeneration (soot oxidation) in the DPF 122 to reduce, minimize, avoid,or optimize reliance on active regeneration. Therefore, under warmconditions, the dual SCR control strategy shifts the burden of NO_(x)reduction toward the underbody SCR system 126, to increase, maximize, oroptimize the amount of NO₂ delivered to the DPF 122.

Finally, overall SCR conversion efficiency can suffer under high exhaustflow conditions such as at high load operation and hard accelerationconditions of the diesel engine. To improve overall system NO_(x)reduction efficiency under such conditions, or decrease the degree towhich such efficiency suffers, the close-coupled SCR 116 and theunderbody SCR 126 may both be used at or near their respective maximumcapacities, under which conditions the close-coupled SCR system 116 maybe considered additional volume to the underbody SCR system 126. Thatis, under some conditions, irrespective of the temperature operatingregime, the close-coupled SCR system 116 is leveraged to reduce theeffective NO_(x) flow into the underbody SCR system 126, to reduce highexhaust flow emissions.

Thus, at some times during operation of a diesel engine, only theclose-coupled SCR system 116 may be used to reduce NO_(x) levels tocomply with tailpipe emissions regulations, while at other times duringoperation of a diesel engine, only the underbody SCR system 126 may beused to reduce NO_(x) levels to comply with tailpipe emissionsregulations, while at yet other times, the burden of reducing NO_(x)levels to comply with tailpipe emissions regulations may be shared bythe two SCR systems 116, 126, such as by any suitable ratio.

For example, the close coupled SCR system 116 may reduce NO_(x) levelsby 10%, 25%, 50%, 75%, or 90% (or any other intermediate percentage) ofthe amount required to comply with tailpipe emissions regulations, whilethe underbody SCR system 126 may reduce NO_(x) levels by a complementaryamount (e.g., 90%, 75%, 50%, 25%, or 10%, respectively) of the amountrequired to comply with tailpipe emissions regulations. Thus, it hasbeen found that it is also valuable to balance the NO_(x) reductionburden between the two SCR systems 116 and 126 to further improveefficiency of operation, to ensure compliance with emissions standardswithout unduly reducing overall fuel efficiency, and to increase,maximize, or optimize NO_(x) reduction per unit DEF utilized.

First, an initial lookup table or database is built or populated underideal or idealized conditions in accordance with standardized laboratoryexperiments. Such experiments may operate a heavy-duty diesel engine atsteady state under a variety of operating conditions to determineproperties of the exhaust gas flow generated by the engine at steadystate under such conditions. For example, for each set of givenoperating conditions, the experiments may measure a mass flow rate ({dotover (m)}_(exh)) of the exhaust gas flow generated by the engine, whichmay be specified in units such as kg/s, determine a resulting molarspecific heat at constant pressure (C_(p)) of the exhaust gas flowgenerated by the engine (which may be unique to each individual enginebut may be expected to be constant over the range of operation of anygiven engine), and measure a resulting exhaust temperature (T₁) of theexhaust gas flow generated by the engine immediately adjacent to anexhaust port or outlet port of the engine itself, such as a turbineoutlet of a turbocharger thereof, which may be measured by the firsttemperature sensor 106 and may be specified in units such as K ordegrees Celsius.

Such experiments may also operate the diesel engine in combination withthe exhaust after-treatment system 100 at steady state under a varietyof operating conditions to determine how the operation of the exhaustafter-treatment system 100 affects properties of the exhaust gas flow asit travels through the exhaust after-treatment system 100 at steadystate under such conditions. For example, for each set of givenoperating conditions, the experiments may use the temperature sensors106, 114, 130, 134, 136, and 138 to measure the temperature of theexhaust gas flow at the locations of the temperature sensors 106, 114,130, 134, 136, and 138, respectively. The resulting measuredtemperatures can be stored in the lookup table or database.

As another example, for each set of given operating conditions, theexperiments may use the first NO_(x) sensor 108 to measure NO_(x) levelsat the location of the first NO_(x) sensor 108, may use the secondNO_(x) sensor 118 to measure NO_(x) levels at the location of the secondNO_(x) sensor 118, and may use such measurements to calculate apercentage reduction in NO_(x) levels between the first and secondNO_(x) sensors 108, 118, which may be taken as a percentage efficiencyof the close coupled SCR system 116. Thus, as noted elsewhere herein,the first NO_(x) sensor 108 and the second NO_(x) sensor 118 can be usedtogether to monitor, assess, or measure the performance of the first SCRsystem 116 at steady state under the various experimental conditions.Similarly, for each set of given operating conditions, the experimentsmay use the second NO_(x) sensor 118 to measure NO_(x) levels at thelocation of the second NO_(x) sensor 118, may use the third NO_(x)sensor 128 to measure NO_(x) levels at the location of the third NO_(x)sensor 128, and may use such measurements to calculate a percentagereduction in NO_(x) levels between the second and third NO_(x) sensors118, 128, which may be taken as a percentage efficiency of the underbodySCR system 126. Thus, as noted elsewhere herein, the second NO_(x)sensor 118 and the third NO_(x) sensor 128 can be used together tomonitor, assess, or measure the performance of the second SCR system 126at steady state under the various experimental conditions. The resultingmeasured NO_(x) levels and calculated percentage efficiencies of the SCRsystems can be stored in the lookup table or database. In someimplementations, the resulting measured NO_(x) levels and calculatedpercentage efficiencies of the SCR systems can be stored in the lookuptable or database as a function of or otherwise correlated with orrelated to the measured temperatures.

As another example, for each set of given operating conditions, theexperiments may monitor the rate at which the first and second DEFinjectors 110, 124 inject DEF into the exhaust gas flow, and may usesuch information in combination with the measurements provided by thefirst, second, and third NO_(x) sensors 108, 118, and 128, to calculateammonia-to-NO_(x) ratios (ANR) at the close-coupled SCR system 116 andat the underbody SCR system 126. The injection rates and calculated ANRscan be stored in the lookup table or database.

As another example, for each set of given operating conditions, theexperiments may monitor the rate at which N₂O is generated and emitted,as well as the state of the DPF 122, the level of passive regenerationthereof that occurs, and the degree to which active regeneration thereofis or would be required. Such information can be stored in the lookuptable or database.

Based on such measurements, calculations, and data stored in the lookuptable or database, ideal, optimal, efficient, or most efficient relativedivisions of the labor or burden of reducing NO_(x) levels to complywith tailpipe emissions regulations between the first, close-coupled SCR116 and the second, underbody SCR 126 may be determined. For example, itmay be determined that it is efficient to allocate the NO_(x) reductionburden between the close-coupled SCR 116 and the underbody SCR 126 basedon the temperature of the catalytic bed of the underbody SCR 126, suchas based on the temperatures of the exhaust gas flow measured by thefifth temperature sensor 134 and/or the sixth temperature sensor 138(e.g., an average thereof).

For example, it may be determined that it is more efficient to allocatea larger portion of the NO_(x) reduction burden, or even all of theNO_(x) reduction burden, to the close-coupled SCR 116 when thetemperatures at the underbody SCR 126 are relatively cold (indicating,for example, that the diesel engine and/or the exhaust after-treatmentsystem 100 are cold or just starting up), and to allocate a largerportion of the NO_(x) reduction burden, or even all of the NO_(x)reduction burden, to the underbody SCR 116 when such temperatures arerelatively hot (indicating, for example, that the diesel engine and/orthe exhaust after-treatment system 100 are hot or operating at or nearsteady-state).

In some embodiments, dividing the labor of reducing NO_(x) levels tocomply with tailpipe emissions regulations includes controlling a rateat which the first DEF injector 110 injects DEF into the exhaust gasflow upstream of the close-coupled SCR system 116 (e.g., as a functionof the NO_(x) levels measured by the first NO_(x) sensor 108) to controlan ANR within the close-coupled SCR system 116 and/or to prevent ammoniaslip from the close-coupled SCR system 116, thereby controllingoperation and SCR reduction efficiency of the close-coupled SCR system116, and operating the second DEF injector 124, the second heater, andthe second, underbody SCR system 126 to further reduce NO_(x) levels toensure compliance with emissions regulations.

As one specific example, it may be determined that when the temperatureof the catalytic bed of the underbody SCR system 126 is 200 degreesCelsius or lower, it is most efficient from a systemic perspective tooperate the first DEF injector 110 to inject DEF into the exhaust gasflow at a minimum rate sufficient to ensure that the ANR in the first,close-coupled SCR is equal to or 100% of the ANR required to reduceNO_(x) levels in the exhaust gas flow just downstream of theclose-coupled SCR system 116 (e.g., as measured by the second NO_(x)sensor 118) to levels in compliance with emissions regulations, that is,such that the first, close coupled SCR system 116 handles the fullNO_(x) reduction burden, and to not begin operating the second DEFinjector 124, the second heater, and the underbody SCR system 126. Itmay further be determined that when the temperature of the catalytic bedof the underbody SCR system 126 is 225 degrees Celsius, it is mostefficient from a systemic perspective to operate the first DEF injector110 to inject DEF into the exhaust gas flow at a minimum rate sufficientto ensure that the ANR in the first, close-coupled SCR is 80% of the ANRrequired to reduce NO_(x) levels in the exhaust gas flow just downstreamof the close-coupled SCR system 116 (e.g., as measured by the secondNO_(x) sensor 118) to levels in compliance with emissions regulations,and to operate the second DEF injector 124, the second heater, and thesecond, underbody SCR system 126 to further reduce NO_(x) levels toensure compliance with emissions regulations.

It may further be determined that when the temperature of the catalyticbed of the underbody SCR system 126 is 275 degrees Celsius, it is mostefficient from a systemic perspective to operate the first DEF injector110 to inject DEF into the exhaust gas flow at a minimum rate sufficientto ensure that the ANR in the first, close-coupled SCR is 60% of the ANRrequired to reduce NO_(x) levels in the exhaust gas flow just downstreamof the close-coupled SCR system 116 (e.g., as measured by the secondNO_(x) sensor 118) to levels in compliance with emissions regulations,and to operate the second DEF injector 124, the second heater, and thesecond, underbody SCR system 126 to further reduce NO_(x) levels toensure compliance with emissions regulations. It may further bedetermined that when the temperature of the catalytic bed of theunderbody SCR system 126 is 300 degrees Celsius, it is most efficientfrom a systemic perspective to operate the first DEF injector 110 toinject DEF into the exhaust gas flow at a minimum rate sufficient toensure that the ANR in the first, close-coupled SCR is 25% of the ANRrequired to reduce NO_(x) levels in the exhaust gas flow just downstreamof the close-coupled SCR system 116 (e.g., as measured by the secondNO_(x) sensor 118) to levels in compliance with emissions regulations,and to operate the second DEF injector 124, the second heater, and thesecond, underbody SCR system 126 to further reduce NO_(x) levels toensure compliance with emissions regulations.

It may further be determined that when the temperature of the catalyticbed of the underbody SCR system 126 reaches an upper threshold orboundary, it is most efficient from a systemic perspective to ceaseoperating the first DEF injector 110, the first heater 112, and theclose-coupled SCR system 116, and to operate the second DEF injector124, the second heater, and the second, underbody SCR system 126 tohandle the full NO_(x) reduction burden and reduce NO_(x) levels toensure compliance with emissions regulations. At temperatures betweenthose identified herein, or at temperatures between any other set oftemperatures so studied or investigated in such experiments, anyinterpolation functions, such as a linear interpolation function, may beused to determine appropriate DEF injection rates. For example, it mayfurther be determined by linear interpolation that when the temperatureof the catalytic bed of the underbody SCR system 126 is 250 degreesCelsius, it is most efficient from a systemic perspective to operate thefirst DEF injector 110 to inject DEF into the exhaust gas flow at aminimum rate sufficient to ensure that the ANR in the first,close-coupled SCR is 70% of the ANR required to reduce NO_(x) levels inthe exhaust gas flow just downstream of the close-coupled SCR system 116(e.g., as measured by the second NO_(x) sensor 118) to levels incompliance with emissions regulations, and to operate the second DEFinjector 124, the second heater, and the second, underbody SCR system126 to further reduce NO_(x) levels to ensure compliance with emissionsregulations. Such information can be stored in the lookup table ordatabase.

Second, during operation of a vehicle, such as a motor vehicle such as aheavy-duty diesel truck, the exhaust after-treatment system 100,including the first, close-coupled SCR system 116 and the second,underbody SCR system 126, may be operated to ensure compliance withemissions regulations while minimizing an incurred fuel penaltyresulting from operation of the components of the exhaustafter-treatment system 100, including the first heater 112 and/or thesecond heater, as described elsewhere herein. In particular, as thetruck and its engine and its engine control unit (“ECU”) are operating,the engine control unit of the truck may continuously monitor how theoperation of the exhaust after-treatment system 100 affects propertiesof the exhaust gas flow as it travels through the exhaustafter-treatment system 100. For example, the ECU may continuouslymeasure or monitor temperatures of the exhaust gas flow at the locationsof the temperature sensors 106, 114, 130, 134, 136, and 138, as well astemperatures of the catalytic beds of the first and second catalyticreduction systems 116, 126.

As another example, the ECU may use the first NO_(x) sensor 108 tomeasure NO_(x) levels at the location of the first NO_(x) sensor 108,may use the second NO_(x) sensor 118 to measure NO_(x) levels at thelocation of the second NO_(x) sensor 118, and may use such measurementsto calculate a percentage reduction in NO_(x) levels between the firstand second NO_(x) sensors 108, 118, which may be taken as a percentageefficiency of the close-coupled SCR system 116. Similarly, the ECU mayuse the second NO_(x) sensor 118 to measure NO_(x) levels at thelocation of the second NO_(x) sensor 118, may use the third NO_(x)sensor 128 to measure NO_(x) levels at the location of the third NO_(x)sensor 128, and may use such measurements to calculate a percentagereduction in NO_(x) levels between the second and third NO_(x) sensors118, 128, which may be taken as a percentage efficiency of the underbodySCR system 126.

As another example, the ECU may monitor the rate at which the first andsecond DEF injectors 110, 124 inject DEF into the exhaust gas flow, andmay use such information in combination with the measurements providedby the first, second, and third NO_(x) sensors 108, 118, and 128, tocalculate ammonia-to-NO_(x) ratios (ANR) at the close-coupled SCR system116 and at the underbody SCR system 126.

Based on such measurements and calculations, in combination with thedata stored in the lookup table or database, the engine control unit ofthe truck may continuously assess or determine a desired, optimal, ormost efficient division of the labor of reducing NO_(x) levels to complywith tailpipe emissions regulations between the first, close-coupled SCR116 and the second, underbody SCR 126. For example, the engine controlunit may continuously use the bed temperature of the underbody SCRsystem 126 in combination with the data stored in the lookup table ordatabase to determine a desired or optimal rate at which to inject DEFinto the exhaust gas flow upstream of the close-coupled SCR system 116.Such a determination may rely on interpolation between data pointsstored in the lookup table or database.

The engine control unit may then operate the first DEF injector 110 toinject DEF into the exhaust gas flow at that rate. The engine controlunit may then operate the second DEF injector 124, the second heater,and the second, underbody SCR system 126 to further reduce NO_(x) levelsto ensure compliance with emissions regulations. The engine control unitmay furthermore continually control and/or adjust or update operation ofthe first DEF injector 110 to inject DEF into the exhaust gas flow at arate that varies over time, such as over at least 15 seconds, 30seconds, one minute, two minutes, or four minutes, and as a function ofan average of the temperature measurements provided by the fifthtemperature sensor 134 and the sixth temperature sensor 138, such asacross a range in such an average temperature of at least 25, 50, 75,100, 125, or 150 degrees Celsius. As such operation of the first DEFinjector 110 varies, the engine control unit may also continuallycontrol and/or adjust or update operation of the second DEF injector124, the second heater, and the second, underbody SCR system 126 tofurther reduce NO_(x) levels to ensure compliance with emissionsregulations.

The systems and techniques described herein can facilitate continuousand simultaneous dosing of DEF onto the catalytic beds of twoindependent SCR systems, namely, the close-coupled SCR system 116 andthe underbody SCR system 126, such as by using two independent DEFinjectors, each located upstream of a respective one of the SCR systems,namely, the first DEF injector 110 and the second DEF injector 124, toincrease overall systemic efficiency while maintaining compliance withNO_(x) emissions regulations. The systems and techniques describedherein can be used to reduce NO_(x) tailpipe emissions to ultra-lowlevels, which may be referred to as “ultra-low NO_(x)” or “ULN” levels.Further, the systems and techniques described herein can be leveraged toreduce especially high NO_(x) emissions, such as on hard accelerationtransients, to within regulated levels. The systems and techniquesdescribed herein may include or use a minimal number of sensors, thatis, the systems and techniques described herein may include or useexactly or no more than a specified number of sensors necessary forperformance as described herein. The systems and techniques describedherein allow efficient operation of the close-coupled SCR system 116and/or the underbody SCR system 126 without ammonia slip, or withminimal ammonia slip, from the SCR systems 116, 126.

FIG. 2 illustrates results of experimental testing of a heavy-dutyvehicle including a diesel engine and the exhaust after-treatment system100. In particular, FIG. 2 illustrates two charts, one on top of theother. In the bottom of the two charts, the horizontal X-axis representstime and the vertical Y-axis represents the instantaneous NO_(x) levels,in units of parts per million, measured by the first, second, and thirdNO_(x) sensors 108, 118, and 128. In the top of the two charts, thehorizontal X-axis represents time on the same scale and interval as inthe bottom of the two charts, and the vertical Y-axis represents thecumulative NO_(x) measured by the first NO_(x) sensor 108 (see the lineindicated by reference numeral 140), the second NO_(x) sensor 118 (seethe line indicated by reference numeral 142), and the third NO_(x)sensor 128 (see the line indicated by reference numeral 144).

As illustrated in FIG. 2, the difference between the NO_(x) levelsmeasured by the first and second NO_(x) sensors 108, 118, and theresulting difference between the cumulative NO_(x) measured by the firstand second NO_(x) sensors 108, 118, rise early in the testing,reflecting the fact that it is generally more efficient to reduce NO_(x)levels near start-up or at cold temperatures at the close-coupled SCR116 than at the underbody SCR 126. As also illustrated in FIG. 2, thedifference between the NO_(x) levels measured by the second and thirdNO_(x) sensors 118, 128, and the resulting difference between thecumulative NO_(x) measured by the second and third NO_(x) sensors 118,128 rise later in the testing, reflecting the fact that it is generallyadvantageous to reduce NO_(x) levels near steady-state operation or athot temperatures at the underbody SCR 126 than at the close-coupled SCR116.

FIG. 3 illustrates results of experimental testing of a heavy-dutyvehicle including a diesel engine and the exhaust after-treatment system100. In particular, FIG. 3 illustrates two charts, one on top of theother. In the bottom of the two charts, the horizontal X-axis representstime and the vertical Y-axis represents the instantaneous rates of DEFdosing, in units of grams per hour, by the first DEF injector 110 andthe second DEF injector 124. In the top of the two charts, thehorizontal X-axis represents time on the same scale and interval as inthe bottom of the two charts, and the vertical Y-axis represents thecumulative ammonia-to-NO_(x) ratio resulting (e.g., from ammoniareleased by the DEF injected by the respective DEF injectors) in theclose-coupled SCR system 116 (see the line indicated by referencenumeral 150) and in the underbody SCR system 126 (see the line indicatedby reference numeral 152).

As illustrated in FIG. 3, the DEF dosing by the first DEF injector 110and the cumulative ammonia-to-NO_(x) ratio resulting in theclose-coupled SCR system rise quickly early in the testing, reflectingthe fact that it is generally more efficient to reduce NO_(x) levelsnear start-up or at cold temperatures at the close-coupled SCR 116 thanat the underbody SCR 126. As also illustrated in FIG. 3, the DEF dosingby the first DEF injector 110 and the cumulative ammonia-to-NO_(x) ratioresulting in the close-coupled SCR system 116 fall, and the DEF dosingby the second DEF injector 124 and the cumulative ammonia-to-NO_(x)ratio resulting in the underbody SCR system 126 rise quickly, later inthe testing, reflecting the fact that it is generally advantageous toreduce NO_(x) levels near steady-state operation or at hot temperaturesat the underbody SCR 126 than at the close-coupled SCR 116.

FIG. 4 illustrates a flow chart 200 of a summarized version of a methodin accordance with the present disclosure. As illustrated in FIG. 4, themethod includes, at 202, operating a diesel engine, thereby generatingan exhaust gas flow. The method further includes, at 204, monitoring atemperature of the exhaust gas flow at an underbody selective catalyticreduction system, and at 206, controlling a DEF injector upstream of aclose-coupled selective catalytic reduction system to inject DEF intothe exhaust gas flow at a rate that varies as a function of themonitored temperature across a range of at least 25 degrees Celsius inthe monitored temperature.

In other embodiments, the exhaust after-treatment system 100 may includethree, four, or any other number of independent SCR systems, togetherwith respective DEF injectors, heaters, temperature sensors, and/orNO_(x) sensors. Each upstream-downstream pair of the SCR systems andrespective DEF injectors, heaters, temperature sensors, and/or NO_(x)sensors can have features corresponding to those described herein forthe upstream close-coupled SCR system 116 and the downstream underbodySCR system 126 and their respective DEF injectors, heaters, temperaturesensors, and/or NO_(x) sensors.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1-20. (canceled)
 21. A method, comprising: operating a diesel engine ofa heavy-duty truck such that the diesel engine generates an exhaust gasflow that enters an exhaust after-treatment system of the heavy-dutytruck, the exhaust after-treatment system including a first selectivecatalytic reduction system and second selective catalytic reductionsystem downstream of the first selective catalytic reduction system withrespect to the exhaust gas flow; monitoring a temperature of the exhaustgas flow at the second selective catalytic reduction system; andcontrolling a diesel emission fluid injector upstream of the firstselective catalytic reduction system to inject diesel emission fluidinto the exhaust gas flow at a rate that varies as a function of themonitored temperature as the monitored temperature changes by at least25 degrees Celsius.
 22. The method of claim 21 wherein controlling thediesel emission fluid injector includes controlling the diesel emissionfluid injector to inject diesel emission fluid into the exhaust gas flowat a rate that varies as a function of the monitored temperature as themonitored temperature changes by at least 50 degrees Celsius.
 23. Themethod of claim 21 wherein controlling the diesel emission fluidinjector includes controlling the diesel emission fluid injector toinject diesel emission fluid into the exhaust gas flow at a rate thatvaries as a function of the monitored temperature as the monitoredtemperature changes by at least 100 degrees Celsius.
 24. The method ofclaim 21, further comprising controlling a diesel emission fluidinjector downstream of the first selective catalytic reduction systemand upstream of the second selective catalytic reduction system toinject diesel emission fluid into the exhaust gas flow, at a rate thatvaries as a function of the monitored temperature, to optimize adivision of labor of reducing NOx levels to comply with emissionsregulations between the diesel emission fluid injector upstream of thefirst selective catalytic reduction system and the diesel emission fluidinjector downstream of the first selective catalytic reduction systemand upstream of the second selective catalytic reduction system.
 25. Themethod of claim 21 wherein the diesel emission fluid injector injectsdiesel emission fluid into the exhaust gas flow at a rate that decreasesas the monitored temperature increases.
 26. The method of claim 21,further comprising monitoring a NOx level upstream of the firstselective catalytic reduction system, and controlling the dieselemission fluid injector includes operating the diesel emission fluidinjector to inject diesel emission fluid into the exhaust gas flow at arate that varies as a function of the monitored NOx level.
 27. Themethod of claim 21 wherein the diesel emission fluid injector injectsdiesel emission fluid into the exhaust gas flow at a rate that isdetermined by reference to a pre-populated lookup table.
 28. The methodof claim 27 wherein the diesel emission fluid injector injects dieselemission fluid into the exhaust gas flow at a rate that is determined byreference to an interpolation function.
 29. The method of claim 21wherein controlling the diesel emission fluid injector includescontrolling the diesel emission fluid injector to inject diesel emissionfluid into the exhaust gas flow at a rate that varies continuously as afunction of the monitored temperature.
 30. The method of claim 21wherein controlling the diesel emission fluid injector includescontrolling the diesel emission fluid injector to inject diesel emissionfluid into the exhaust gas flow at a rate that varies based on ameasured efficiency of the first selective catalytic reduction system.31. The method of claim 21 wherein controlling the diesel emission fluidinjector includes controlling the diesel emission fluid injector toinject diesel emission fluid into the exhaust gas flow at a rate thatvaries based on a measured efficiency of the second selective catalyticreduction system.
 32. The method of claim 21 wherein the diesel emissionfluid injector initially injects diesel emission fluid into the exhaustgas flow at a rate sufficient for the first selective catalyticreduction system to reduce NOx levels to comply with emissionsregulations and, after the diesel emission fluid injector injects dieselemission fluid into the exhaust gas flow at the rate sufficient for thefirst selective catalytic reduction system to reduce NOx levels tocomply with emissions regulations, the diesel emission fluid injectorinjects diesel emission fluid into the exhaust gas flow at a lower ratesufficient for the first selective catalytic reduction system to reduceNOx levels halfway to compliance with emissions regulations.
 33. Themethod of claim 32, further comprising, while the diesel emission fluidinjector upstream of the first selective catalytic reduction systeminjects diesel emission fluid into the exhaust gas flow at the ratesufficient for the first selective catalytic reduction system to reduceNOx levels halfway to compliance with emissions regulations, controllinga diesel emission fluid injector downstream of the first selectivecatalytic reduction system and upstream of the second selectivecatalytic reduction system to inject diesel emission fluid into theexhaust gas flow at a rate sufficient for the second selective catalyticreduction system to reduce NOx levels to comply with emissionsregulations.
 34. The method of claim 32 wherein, after the dieselemission fluid injector injects diesel emission fluid into the exhaustgas flow at the lower rate sufficient for the first selective catalyticreduction system to reduce NOx levels halfway to compliance withemissions regulations, the diesel emission fluid injector ceases toinject diesel emission fluid into the exhaust gas flow.
 35. The methodof claim 34, further comprising, once the diesel emission fluid injectorupstream of the first selective catalytic reduction system ceases toinject diesel emission fluid into the exhaust gas flow, controlling adiesel emission fluid injector downstream of the first selectivecatalytic reduction system and upstream of the second selectivecatalytic reduction system to inject diesel emission fluid into theexhaust gas flow at a rate sufficient for the second selective catalyticreduction system to reduce NOx levels to comply with emissionsregulations.
 36. A method, comprising: operating a diesel engine of aheavy-duty truck such that the diesel engine generates an exhaust gasflow that enters an exhaust after-treatment system of the heavy-dutytruck, the exhaust after-treatment system including a first selectivecatalytic reduction system and a second selective catalytic reductionsystem downstream of the first selective catalytic reduction system withrespect to the exhaust gas flow; monitoring a temperature of the exhaustgas flow at the second selective catalytic reduction system; andcontrolling a diesel emission fluid injector upstream of the firstselective catalytic reduction system to change a rate of injection ofthe diesel emission fluid into the exhaust gas flow a first time whenthe monitored temperature is a first temperature and to change the rateof injection of the diesel emission fluid into the exhaust gas flow asecond time when the monitored temperature is a second temperature thatdiffers from the first temperature by at least 25 degrees Celsius. 37.The method of claim 36 wherein the second temperature differs from thefirst temperature by at least 50 degrees Celsius.
 38. The method ofclaim 36 wherein the second temperature differs from the firsttemperature by at least 100 degrees Celsius.
 39. A heavy-duty truck,comprising: a diesel engine; an exhaust after-treatment system having anupstream end and a downstream end opposite the upstream end, theupstream end coupled to the diesel engine, the exhaust after-treatmentsystem including a first selective catalytic reduction system and asecond selective catalytic reduction system downstream of the firstselective catalytic reduction system; and an engine control unitconfigured to: operate the diesel engine such that the diesel enginegenerates an exhaust gas flow that enters the exhaust after-treatmentsystem; monitor a temperature of the exhaust gas flow at the secondselective catalytic reduction system; and control a diesel emissionfluid injector upstream of the first selective catalytic reductionsystem to inject diesel emission fluid into the exhaust gas flow at arate that varies as a function of the monitored temperature as themonitored temperature changes by at least 25 degrees Celsius.
 40. Theheavy-duty truck of claim 39 wherein the engine control unit isconfigured to control the diesel emission fluid injector to injectdiesel emission fluid into the exhaust gas flow to achieve a targetammonia-to-NOx ratio in the first selective catalytic reduction system.